1. Field of the Invention
The present invention relates to a white balance control device of a video camera and a video still camera, and particularly to a white balance control device for properly controlling white balance by a method of inside light measurement without color failure occurring, a white balance control device for accurately controlling white balance by reducing the white balance converging period, a white balance control device for properly controlling white balance control by a method of inside light measurement without color failure occurring when photographing indoors and outdoors, a white balance control device with a device for selecting a light source by manual operation in order to prevent deterioration of reproduced colors caused by a difference between a data of a selected light source and an acutal color temperature of the selected light source.
2. Related Art
In conventional video cameras and video still cameras, a white balance control is utilized in order to reproduce a white object as a white colored material. The white balance control is operated by controlling a gain of a red signal circuit and a gain of a blue signal circuit in the camera based on a Green signal as a reference value.
In the white balance control, it is necessary to measure a tint (color temperature) of a photographing working field for operating the white balance control. As methods of the white balance control, there are an outside light measuring method and an inside light measuring method, which are different from each other in measuring the color temperature of the photographing working field.
In the outside light measuring method, a color temperature is directly detected by a color temperature sensor. A white balance control signal for a red signal and a white balance control for a blue signal are produced based on reference data detected by the color temperature sensor in order to control the white balance. The color temperature sensor is integrally formed with photosensors, for example, a photo sensor with a red filter, a photo sensor with a Green filter and a photo sensor with a blue filter. The white balance control for the blue signal are produced by an output voltage of each photo sensor, respectively.
In the inside light measuring method, a color temperature is indirectly detected by a color temperature sensor. If a white balance is matching in a picture, an averaged color of whole colors in a picture becomes achromatic color (grey). The inside light measuring method utilizes this theory. That is, an integrated averaged value of color difference signals R-Y and B-Y at a reference color temperature that an averaged color for all of the colors in a picture becomes an achromatic color is designated as a reference value for each signal. Thus, Gains of the red signal and the blue signal are controlled in order to match the integrated averaged value with the reference value.
The above described values of the white balance control for a red signal and the white balance control for a blue signal are calculated by a device, such as a microcomputer and then white balance control signals for a red signal and a blue signal are output from the microcomputer to a white balance circuit. In the white balance circuit, gains of a red signal and a blue signal are controlled based on values of the white balance control signals.
On the other hand, in a manual white balance method, an operator detects a kind of light source (for example, sunshine, electric bulb, fluorescent lamp and so on) in a photographing field by himself and then the operator selects the kind of the light source by manually operating a device for selecting a light source, such as a switch. In accordance with a selection of the light source, Gains of the red signal circuit and the blue signal circuit are set at specific gains corresponding to the kinds of the light sources.
The condition in which an averaged color becomes an achromatic color (grey) in a picture is achieved when the video camera takes a photograph of an ordinal sight in which various colors are mixed randomly. However, when the video camera takes a photograph of an object having a background of a blue sky, a blue ocean or a red wall, the above described condition cannot be accomplished. An averaged color for all of the colors in a picture is not an achromatic color, the averaged color becomes a color with a blue tint or a red tint. If the video camera takes a photograph of a sight dominated by a mono-colored background, the averaged color of the picture is recognized as the achromatic color by a white balance control in the inside light measuring method, although an averaged color in a picture is not an achromatic color. As a result, a reference white level is only slightly different from the true white level. The color of the background is discolored and a color of a main object (person) is controlled to be shifted to its additive complementary color (additive complementary color against the background), and a so called "color failure" occurs.
As a result, in an inside light measurement control, a video camera with a white balance control unit takes a photograph of an ordinal sight in which various colors are mixed randomly, so that the white balance control is operated properly. On the other hand, when the video camera takes a photograph of a specific sight in which a specific color dominates, the above described color failure occurs.
In a conventional automatic white balance control device, white balance control signals are output with predetermined intervals. If the predetermined intervals of the output white balance control signals are too short, the white balance control signals are not accurately converged by the occurrence of hunting. On the other hand, if the predetermined intervals of the output balance control signals are too long, the device spends a large amount of time to converge, although the white balance control signals are accurately converged. Therefore, some of the following problems arise for example. Such as, if a color temperature is suddenly changed or a light source is turned on, the device needs to spend a long time for operating white balance normally in the case when the white balance control signals should be converged as early as possible.
In the above described conventional manual white balance method, gains of the red signal circuit and the blue signal circuit are set to the specific value which depends on the kinds of the light sources. Even if the same kind of the light source is selected, a deterioration of reproduced colors occurs by a difference between a predetermined value of a selected light source and an actual color temperature, because there are various fluorescent lamp types and colors associated with a camera which have undesirable influences.